Edward J. Olsen

Oxygen isotope studies of ordinary chondrites

Several stages in the evolution of ordinary chondritic meteorites are recorded in the oxygen isotopic composition of the meteorites and their separable components (chondrules, fragments, clasts, and matrix). The whole-rock isotopic compositions reflect the iron-group of the meteorite (H, L, or LL). Isotopic uniformity of H3 to H6 and L3 to L6 are consistent with closed-system metamorphism within each parent body. LL3 chondrites differ slightly from LL4 to LL6, implying a small degree of opensystem aqueous alteration and carbon reduction. On the scale of individual chondrules, the meteorites are isotopically heterogeneous, allowing recognition of the solar-nebular processes of chondrule formation. Chondrules for all classes of ordinary chondrites are derived from a common population, which was separate from the population of chondrules in carbonaceous or enstatite chondrites. Chondrules define an isotopic mixing line dominated by exchange between 16O-rich and 16O-poor reservoirs. The oxygen isotopic compositions of chondrites serve as “fingerprints” for identification of genetic association with other meteorite types (achondrites and iron) and for recognition of source materials in meteoritic breccias.

Origin of the high-temperature fraction of C2 chondrites

The coarse-grained fraction of C2 chondrites is composed mostly of single crystals and aggregates of crystals of Mg-rich olivine and pyroxene. They do not possess compelling textural evidence of being the solidification products of rapidly-quenched molten droplets. Metal inclusions in the silicates contain 3·82–8·88 mole% Ni, 0·16–0·70 per cent Co, 0·17–1·07 per cent Cr and up to 5·70 per cent P. Thermodynamic calculations show that alloys of these compositions may be condensates from the solar nebula. The implication is that the high-temperature fraction of C2 chondrites consists mostly of high-temperature condensates. Chemical data show that the high-temperature fraction has an Fe/Mg atomic ratio of ⩽ 0·31 compared to 1·3 in the matrix, indicating that much of the iron has been lost from the high-temperature fraction and converted to the troilito and oxidized iron of the low-temperature fraction. The presence of low-Ni metal grains in the aggregates and high Ni/Fe and Co/Fe ratios in the matrix of some C2s indicates preferential loss of early NiCo-rich metal from the high-temperature fraction during condensation.

Mineralogy and petrology of silicate inclusions in iron meteorites

Silicate inclusions in 17 iron meteorites have been analyzed by the electron microprobe and classified, according to their phase assemblages, compositions, and textures, into three major types: Odessa, Copiapo, and Weekeroo Station, and three miscellaneous types: Enon, Kendall County, and Netschaëvo. Phase compositions in both Odessa- and Copiapo-type inclusions are very similar, but the two types are different in texture and constituent phases. Weekeroo Station-type inclusions are very different in every respect from other inclusions.For Odessa- and Copiapo-type inclusions, the distribution coefficients of Fe2+ and Mg in coexisting orthopyroxene and clinopyroxene indicate equilibration temperatures of ∼1,000° C, and the Ca/(Ca+Mg) ratios indicate temperatures of 900° C to 1,000° C. Equilibration temperatures determined for chromite-olivine pairs have a higher range of 1,154° C to 1,335° C. Minor element distributions among coexisting ferromagnesian silicates in these inclusions follow consistent patterns and are constant for any given sample, suggesting equilibrium assemblages. Major and minor element distributions for Weekeroo Station inclusions are anomalous, indicating nonequilibrium.Compositional data, the fragmentary shapes of many inclusions, the highly differentiated characteristic of two types of inclusions, the apparent disequilibrium between kamacite in inclusions and kamacite of the iron host, and the relict chondrules found in Netschaëvo suggest that many of the inclusions did not form cogenetically with the iron host, but represent pre-existing stony material that was taken up by an iron melt, probably not in the core of the parent body (or bodies).

Refractory inclusions in the Murchison meteorite

Mineralogical and petrographic studies of a wide variety of refractory objects from the Murchison C2 chondrite have revealed for the first time melilite-rich and feldspathoid-bearing inclusions in this meteorite, but none of these is identical to any inclusion yet found in Allende. Blue spinel-hibonite spherules have textures indicating that they were once molten, and thus their SiO2-poor bulk composition requires that they were exposed to higher temperatures (>1550°C) than those deduced so far from any Allende inclusion. Melilite-rich inclusions are similar to Allende compact Type As, but are more Al-, Ti-rich. One inclusion (MUCH-1) consists of a delicate radial aggregate of hibonite crystals surrounded by alteration products, and probably originated by direct condensation of hibonite from the solar nebular vapor. The sinuous, nodular and layered structures of another group of inclusions, spinel-pyroxene aggregates, suggest that these also originated by direct condensation from the solar nebular gas. Each type of inclusion is characterized by a different suite of alteration products and/or rim layers from all the other types, indicating modification of the inclusions in a wide range of different physico-chemical environments after their primary crystallization. All of these inclusions contain some iron-free rim phases. These could not have formed by reaction of the inclusions with fluids in the Murchison parent body because the latter would presumably have been very rich in oxidized iron. Other rim phases and alteration products could have formed at relatively low temperatures in the parent body, but some inclusions were not in the locations in which they were discovered when this took place. Some of these inclusions are too fragile to have been transported from one region to another in the parent body, indicating that low temperature alteration of these may have occurred in the solar nebula.

Oxygen isotope relationships in iron meteorites

Iron meteorites with oxygen-bearing phases can be classified in terms of their oxygen isotopic abundances. These iron meteorite classes are isotopically similar to various stony meteorite classes, which may indicate a common origin. The group IAB and IIICD irons may be related to the winonaites; group IIE irons may be related to H chondrites; group IVA irons may be related to L or LL chondrites.

Late Devonian “Kellwasser Event” mass-extinction horizon in Germany: No geochemical evidence for a large-body impact

The hypothesis that the Late Devonian (Frasnian-Famennian) mass extinction was triggered by an asteroidal impact has received renewed attention with the discovery of a Late Devonian Ir anomaly in Australia. In Europe, the mass-extinction event corresponds stratigraphically to the geographically widespread Kellwasser black-shale and bituminous limestone units, and the biological crisis itself has been alternatively designated the Kellwasser Event. The authors report here the results of an extensive geochemical analysis of the Kellwasser stratigraphic interval in a section with exceptional conodont zonal control in the Federal Republic of Germany. No Ir anomaly was found, neither at the biological crisis horizon recognized in Europe nor at the conodont horizon that corresponds to the Ir anomaly zone reported in Australia. No shock-metamorphosed quartz, sanidine spherules, or siderophile-rich magnetic spherules were found, which might have been indicative of a cometary impact. Oxygen-isotope ratios show little variation across the mass-extinction horizon, though carbon-isotope data suggest a sudden increase in phytoplankton activity. They further note that the Australian Ir anomaly (1) is most likely not associated with a large-body impact because no equivalent Ir signature occurs in Europe and (2) is stratigraphically above the European biological crisis horizon, thus postdating the Kellwasser mass-extinction event.

Antarctica: A Deep-Freeze Storehouse for Meteorites

Meteorites thit fall on the Antarctic ice cap are preserved for long periods of time under very clean conditions as they are carried toward the continental margin. If the host ice encounters a barrier it cannot flow over or around, it tends to dissipate by ablation, leaving an accumulation of meteorites on the surface.

Chondrules: First Occurrence in an Iron Meteorite

Complete chondrules and fragments of chondrules have been found within silicate inclusions from the octahedrite iron meteorite Netschaevo. The bulk chemical composition, mineralogy, and mineral chemistry indicate that this chondritic material has properties intermediate between those of the H-group chondrites and those of the enstatite chondrites.

Petrologic History of Moon Suggested by Petrography, Mineralogy, and Crystallography

Opaque mineral compositions indicate that the fugacity of oxygen is approximately 10-13 (earth basalts, 10-10). Experiments under reducing conditions suggest that the crystallization range is approximately 1140� to 1070�C. Iron-rich pyroxmangite, fayalite, and hedenbergite occur in microgabbro. Ferropseudobrookite rimmed by ilmenite containing rutile and Cr-spinel lamellae occurs in ferrobasalt. Plagioclase vitrophyres in breccia can explain highland Surveyor VII analysis. We suggest crystal-liquid differentiation of out-gassed convecting moon with growing Fe-rich core, olivine-pyroxene mantle, plagioclase-rich dynamic crust underlain by nonspherical, inversely stratified ferrobasalt. Impact-breaking or convection-thrusting of crust releases fraction rich in Fe and Ti. Scanning electron microscopy of glass balls reveals minute depressions consistent with micrometeorite impact.

On the origin of isolated olivine grains in type 2 carbonaceous chondrites

Abstract The origin of olivine grains isolated in the matrix of C2 carbonaceous chondrites is an important problem. If these grains are condensates from a solar nebular gas, they contain compositional, isotopic and physical features that further elucidate that process. If, however, they are grains released by the breakup of chondrules, then many important condensation features have been lost during the melting that took place to form chondrules. In evaluating these two possibilities, care must be taken to determine which inclusions in C2 meteorites are actual chondrules and which are aggregates of grains that have never undergone melting. The two main types of aggregates, pyroxene-rich and pyroxene-poor, are forty to fifty times more abundant than chondrules. Four scenarios are presented to account for the kinds of aggregates and isolated grains seen in the Murchison C2 meteorite. An analysis of these scenarios is made in light of olivine crystal morphology, comparison of composition of glass inclusions inside olivine grains with interstitial glass in true chondrules and size distributions of olivines, isolated, in aggregates and in chondrules. It is concluded that no scenario that includes a chondrule-making step can account for the observed population of isolated olivine grains. An origin by direct condensation, partial comminution, aggregation and accretion best accounts for the sizes and morphological features observed.